This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.

Unbound threads in a process may or may not be required to be
simultaneously active. By default, the threads implementation ensures
that a sufficient number of threads are active so that the process
can continue to make progress. While this conserves system resources,
it may not produce the most effective level of concurrency.
The pthread_setconcurrency() function allows an application to inform
the threads implementation of its desired concurrency level,
new_level. The actual level of concurrency provided by the
implementation as a result of this function call is unspecified.
If new_level is zero, it causes the implementation to maintain the
concurrency level at its discretion as if pthread_setconcurrency()
had never been called.
The pthread_getconcurrency() function shall return the value set by a
previous call to the pthread_setconcurrency() function. If the
pthread_setconcurrency() function was not previously called, this
function shall return zero to indicate that the implementation is
maintaining the concurrency level.
A call to pthread_setconcurrency() shall inform the implementation of
its desired concurrency level. The implementation shall use this as
a hint, not a requirement.
If an implementation does not support multiplexing of user threads on
top of several kernel-scheduled entities, the
pthread_setconcurrency() and pthread_getconcurrency() functions are
provided for source code compatibility but they shall have no effect
when called. To maintain the function semantics, the new_level
parameter is saved when pthread_setconcurrency() is called so that a
subsequent call to pthread_getconcurrency() shall return the same
value.

If successful, the pthread_setconcurrency() function shall return
zero; otherwise, an error number shall be returned to indicate the
error.
The pthread_getconcurrency() function shall always return the
concurrency level set by a previous call to pthread_setconcurrency().
If the pthread_setconcurrency() function has never been called,
pthread_getconcurrency() shall return zero.

The pthread_setconcurrency() function shall fail if:
EINVAL The value specified by new_level is negative.
EAGAIN The value specified by new_level would cause a system resource
to be exceeded.
The pthread_setconcurrency() function shall not return an error code
of [EINTR].
The following sections are informative.

Application developers should note that an implementation can always
ignore any calls to pthread_setconcurrency() and return a constant
for pthread_getconcurrency(). For this reason, it is not recommended
that portable applications use this function.

Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) In the event of any discrepancy between this version and
the original IEEE and The Open Group Standard, the original IEEE and
The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 PTHREAD_GETCONCURRENCY(3P)